Hardfacing rock bit cones for erosion protection

ABSTRACT

A roller cone for a rock bit is disclosed. The cone includes a cone substrate and hardfacing applied by a welding process in grooves on the exterior surface of the cone substrate. The cone also includes hardfacing applied by a thermal spraying process to insert lands on the exterior surface of the cone substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation in part of application Ser. No. 09/235,257 filed on Jan. 22, 1999, entitled, “Hardfacing Rock Bit Cones for Erosion Protection”, and assigned to the assignee of the present invention, which application itself claims priority from U.S. Provisional application Ser. No. 60/072,276 filed on Jan. 23, 1998.

FIELD OF THE INVENTION

[0002] The invention relates to drill bits used to drill earth formations, and more particularly to wear protection for drill bit roller cones.

BACKGROUND OF THE INVENTION

[0003] Drilling earth formations is commonly performed using rotary drill bits called “rock bits”. Rock bits usually have a plurality of roller cones are rotatably mounted on journals which form part of a bit body. The roller cones rotate about their respective axes on each journal and are, in turn, rotated about the main axis of the drill bit, as the bit is rotated by a drill string or drilling motor.

[0004] In drilling boreholes for oil and gas wells, blast holes, raise holes and the like, rock bits generally operate in a highly abrasive environment. This abrasive environment exists during drilling operations even with the use of a medium for cooling, circulating, and flushing the borehole. The medium may be either drilling mud, air, or another liquid or gas.

[0005] When drilling hard earth formations, a rock bit which has tungsten carbide inserts projecting from the body of the roller cones is generally is used, due to the inserts' relative hardness. However, tungsten carbide inserts are mounted in a relatively soft metal (e.g. steel) that forms the body of the roller cone. This relatively soft cone body may be abraded or eroded away when subjected to the highly abrasive drilling environment. The abrasion or erosion occurs primarily due to the presence of relatively fine cuttings and chips from the formation that are in the borehole. Additional causes of abrasion and erosion include the direct blasting or washing effect of the drilling fluid used in the drilling process, and roller or sliding contact of the cone body with the formation.

[0006] When the portion of the cone material that supports the inserts is substantially eroded or abraded away, drilling forces on the bit either may break the inserts or may force them out of the roller cone body. As a result, the bit is no longer effective in cutting the formation. Moreover, the inserts that break off from the roller cone may further damage other inserts, the roller cones, or other parts of the bit, eventually leading to a catastrophic failure.

[0007] Erosion of the roller cone body usually is most pronounced on the inner and outer edges of the lands of the cone surface. The edges of the lands are immediately adjacent to the insert and the groove between two rows of inserts. The greatest wear on the roller cone surface lands is usually on the inner edges of the outer rows and on the outer edges of the inner rows of inserts. When drilling relatively soft but abrasive formations, the bit is able to penetrate at an extremely high rate. This can result in individual cutting inserts penetrating entirely into the formation, causing the formation to come into contact with the cone body. When such contact occurs, the relatively soft cone shell material can wear away at the edges of the surface lands until the interior portion of the insert becomes exposed. The insert retention ability of the cone body is thus reduced, which may result in the loss of the insert and reduction of bit life. Because the penetration rate is related to the condition of the bit, the drill bit life and efficiency are of great importance in the drilling of boreholes. Accordingly, various methods of hardfacing rock bit cones for erosion or abrasion protection have been attempted. For example, thermal spraying has been used to coat all the exposed cone surfaces, including the inserts, with a hardfacing material. Another method includes placing small, flat-top compacts of hard material in the wear prone cone shell areas to prevent cone erosion. Since erosion of groove surface can be the main cause of insert loss due to erosion, methods were developed to apply hardfacing material to both the lands and the grooves of a roller cone.

[0008] It should be noted that inserts are typically retained in a roller cone by the hoop tension generated when the insert is press fit into a drilled hole in the roller cone body. Accordingly, any method to alleviate the erosion of the roller cone must take into consideration that the hoop tension holding the insert must be retained. It has been found to be undesirable to press the inserts into the cutter before applying hardfacing material, because using of heat to adhere the hardfacing material to the surface of the roller cone relieves the stresses (including the hoop tension) in the roller cone. Therefore, it is more desirable to apply hardfacing material to both the lands and grooves of a roller cone surface for erosion protection before the insert holes are drilled.

[0009] For the foregoing reasons, there exists a need for an effective yet economical method of applying hardfacing material to roller cone surfaces for effective erosion protection. To reduce the cost of manufacturing such roller cone bits with hardfacing material, it is desirable that the method not be complicated and tedious. Further, the hardfacing material should be applied to the roller cone surfaces before the insert holes are drilled.

SUMMARY OF THE INVENTION

[0010] One aspect of the invention is a roller cone for a rock bit. The roller cone according to this aspect includes a cone substrate and hardfacing applied by a welding process in grooves on the exterior surface of the cone substrate. The roller cone also includes hardfacing applied by a thermal spraying process to insert lands on the exterior surface of the cone substrate. In some embodiments, the hardfacing includes tungsten carbide. In some embodiments, the welding process includes plasma transferred arc welding. In some embodiments, the thermal spraying process includes one or more of HVOF, HAOF and D-gun spraying.

[0011] Another aspect of the invention is a rock bit including a bit body having at least one journal depending therefrom. A roller cone is rotatably mounted on the journal. The roller cone includes a cone substrate and hardfacing applied by a welding process in grooves on the exterior surface of the cone substrate. The roller cone also includes hardfacing applied by a thermal spraying process to insert lands on the exterior surface of the cone substrate. In some embodiments, the hardfacing includes tungsten carbide. In some embodiments, the welding process includes plasma transferred arc welding. In some embodiments, the thermal spraying process includes one or more of HVOF, HAOF and D-gun spraying.

[0012] Another aspect of the invention is a method for hardfacing a roller cone for a drill bit. A method according to this aspect of the invention include applying hardfacing by a welding process to grooves between insert lands on a roller cone body, and applying hardfacing by a thermal spraying process to the insert lands. In some embodiments, the hardfacing includes tungsten carbide. In some embodiments, the welding process includes plasma transferred arc welding. In some embodiments, the thermal spraying process includes one or more of HVOF, HAOF and D-gun spraying.

[0013] Other aspects and advantages of the invention will be apparent from the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a perspective view of a prior art roller cone bit.

[0015]FIG. 2 is a cross-sectional view of a prior art cone at the bottom of a borehole.

[0016]FIG. 3 is a cross-sectional view of a cone undergoing a hardfacing process according to one embodiment.

[0017]FIG. 4 is a cross-section of a cone with hardfacing material applied to the surface of the cone according to another embodiment.

[0018]FIG. 5 is an isometric view of a rock bit with three cones overlaid with hardfacing material according to still another embodiment.

[0019]FIG. 6 is a schematic of a plasma transferred arc process in accordance with an embodiment.

[0020]FIG. 7 is a schematic of a gas-shielding tungsten arc process in accordance with an embodiment.

[0021]FIG. 8 is a schematic of a metal-inert gas arc welding process in accordance with an embodiment.

[0022]FIG. 9 is a cross-sectional view of a cone in a 7-7/8 inch mining rock bit coated with hardfacing material according to one embodiment.

[0023]FIG. 10 shows an example of a roller cone having hardfacing applied by two different techniques.

DETAILED DESCRIPTION

[0024] Embodiments of the invention provide a hardfacing coating that exhibits good erosion resistance and possesses strong metallurgical bonding with a roller cone surface. The hardfacing is applied by an arc process to at least part of the cone surface.

[0025]FIG. 1 illustrates a typical prior art rock bit for used for drilling boreholes in earth formations. The rock bit 10 has a steel body 20 with threads 14 formed at an upper end and three legs 22 at a lower end. The threads 14 are adapted to couple the bit 10 to a drill string or drilling too assembly (not shown) as in known in the art.

[0026] Each of three roller cones 16 is rotatably mounted on a corresponding leg 22 at the lower end of the body 20. A plurality of inserts 18, which are typically made from cemented tungsten carbide are press-fit or interference fit into insert sockets (not shown separately) formed in the cones 16. Lubricant is provided to the journals (19 in FIG. 2) on which the cones are mounted from grease reservoirs 24 in the body 20. This configuration generally is used for sealed-bearing rock bits. For open-bearing (unsealed) rock bits, such as those typically used in mining applications, there typically are no grease reservoirs 24.

[0027] Referring to FIG. 2, when in use, the rock bit is threaded onto the lower end of a drill string (not shown) and lowered into a well or borehole. The drill string is rotated by a rig rotary table (not shown) with the carbide inserts 18 in the cones 16 engaging the bottom and side of the borehole 25. As the bit rotates, the cones 16 rotate on the bearing journals 19 and essentially roll around the bottom of the borehole 25. The weight on the bit is applied to the rock formation by the inserts 18 and the rock is crushed and chipped by the inserts. A drilling fluid is pumped through the drill string to the bit and is ejected through nozzles 26 (shown in FIG. 1). The drilling fluid then travels up the annulus formed between the exterior of the drill pipe and the borehole 25 wall, carrying with it most of the cuttings and chips. In addition, the drilling fluid serves to cool and clean the cutting end of the bit as it works in the borehole 25.

[0028]FIG. 2 also shows the lower portion of the leg 22 which supports a journal bearing 19. A plurality of cone retention balls (“locking balls”) 21 and roller bearings 12 a and 12 b surround the journal 19. An O-ring 28, located within in an O-ring groove 23, seals the bearing assembly. The type of seal and roller cone retention device are only shown here to illustrate the general structure of a rock bit and are not intended to limit the invention.

[0029] The cone 16 includes multiple rows of the inserts, and has a heel portion 17 located between gage row inserts 15 and the O-ring groove 23. A plurality of protruding heel row inserts 30 are about equally spaced around the heel 17. The heel row inserts 30 and the gage row inserts 15 act together to cut the gage diameter of the borehole 25. The interior row inserts 18 generally are arranged in concentric rows and they serve to crush and chip the earth formations being drilled.

[0030] As used herein, the term “erosion” refers to both erosion and other abrasive wear. Much of the erosion of the cone body typically occurs between the gage row inserts 15 and heel row inserts 30. Furthermore, erosion also may occur at the lands 27 between the gage row inserts 15 and inner row inserts 18. Generally, a “land” refers to a surface on a roller cone where holes (“sockets”) are drilled to mount the inserts. It is also possible that erosion may occur in the grooves 24 between successive inner row inserts 18. These areas on a roller cone surface are collectively referred to as “areas susceptible to erosion.” Erosion in these areas may result in damage to the cone, loss of the inserts and/or cone cracking, particularly between the inserts. In highly erosive environments, the whole cone body may be subjected to severe erosion.

[0031] Some of the embodiments of the invention provide rock bits with hardfacing coating in the cone areas susceptible to erosion. The cone is generally made according to the following steps: (1) determining the areas susceptible to erosion on the cone surface when the rock bit is in use; (2) depositing a layer of hard-facing material by an arc process in the areas susceptible to erosion; (3) heat treating the cone after the deposition of hardfacing material; (4) drilling sockets for receiving inserts on the conical surface in areas that are substantially away from the areas overlaid with a layer of hardfacing material; and (5) pressing inserts into the sockets on the roller cone. In some embodiments, as will be further explained, the cone making process can be improved by arc-process depositing hardfacing in the cone grooves, and after heat treating and drilling the sockets, applying hardfacing to the insert land surfaces by a thermal spraying process.

[0032] Here, “substantially away” means a separation of at least {fraction (1/16)} inch. Optionally, the cone may be annealed before the hardfacing material is deposited. This annealing step may reduce crack initiation in the cone surface area affected by heat during the hardfacing process.

[0033] In some embodiments, the location and arrangement of the inserts may be determined first. Afterwards, the areas which are susceptible to erosion on the cone surface are determined. As illustrated in FIG. 3, a layer of hardfacing material is then deposited on the identified areas. FIG. 3 shows a cone 16 before the insert-receiving holes are drilled and inserts are press-fitted therein. The intended locations of the inserts are represented by dotted lines. An arc torch 40 is generally placed at a predetermined distance from the surface of the cone. A layer of hardfacing material 50 is contained within and deposited by an arc flame 44. The torch 40 may be moved along the surface of the roller cone to deposit the layer of hardfacing material in all of the desired areas. After areas susceptible to cone erosion have been overlaid, the cone is heat treated according to methods well known in the art. After heat treatment, holes or sockets are drilled in the predetermined locations on the cone and the inserts are press-fit into the sockets.

[0034] After the manufacture of cone 16 is completed, the cone is mounted on journal 19 as illustrated in FIG. 4. A layer of hardfacing material is shown deposited in different areas of the cone 50 a, 50 b, and 50 c that are prone to erosion. The layers 50 a, 50 b and 50 c may be of the same or different hardfacing materials, depending upon the application of the rock bit.

[0035]FIG. 5 shows a rock bit with three cones 16 a, 16 b, and 16 c overlaid with hardfacing material according to another embodiment. Although the insert configuration on one cone is different from that of other cones in FIG. 5, it is entirely acceptable to manufacture a rock bit with three identical cones. This figure indicates that additional hardfacing layer 50 d may be deposited in the lands 27 between gage row inserts 15. It should be understood that the shape of the hardfacing layer 50 is not critical so long as the boundary of the hardfacing layer is substantially away from the inserts 15 and 18. Consequently, various shapes of hardfacing are possible, including, but are not limited to: rounded, circular, elliptical, square, rectangular, trapezoidal, oblong, arched, triangular, annular, or any other suitable regular or irregular shape. A layer of hardfacing material also may be deposited in the grooves of the roller cone as a continuous circumferential ring.

[0036] In a typical rock bit, the nose of a roller cone is situated close to the nose of one or more of the other cones. As a result, there is limited clearance between the noses. To avoid an undesirable reduction in the clearance between the noses, it may be desirable to make a groove or recess in the areas susceptible to erosion at the nose of the roller cone. As shown in FIG. 4, a layer of hardfacing material 50 a is deposited in the groove so that the hardfacing material is substantially flush with the surface of the cone. In this way, the nose area of the roller cone is protected from erosion without sacrificing clearance between the noses of the roller cones. It should be understood that a groove as shown in this embodiment may be placed in other suitable areas of the cone surface, and the position of the groove is not necessarily limited to the nose region of a cone.

[0037] Hardfacing material may be deposited, as will be further explained, particularly in the groove areas, by any one of a number of arc processes that are known in the art. Here, an “arc process” refers to a hardfacing process that uses an arc between an electrode and a work piece to be hardfaced. One such process is the plasma transferred arc (PTA) welding process. As shown in FIG. 6, the PTA welding process uses a torch similar to a conventional plasma arc torch with an electrode grounded to the work piece. A PTA system generally includes two power supplies: a pilot arc power supply 65 and a transferred arc power supply 66. In a PTA welding process, a pilot plasma arc is initiated between a tungsten or tungsten-thorium electrode 62 and a copper orifice 67 with a water-cooled electrode 61. An inert gas 63, such as argon, flowing through the orifice is ionized so that it initiates a secondary arc between the tungsten electrode 62 and the work piece (i.e., the roller cone) 16 when the current is increased. The arc and the weld zone are shielded by a gas 60 flowing through an outer nozzle 68. The shielding gas may include argon, helium or mixtures of inert gases. The plasma created by the arc current may be further collimated by nozzle 68 and then expanded and accelerated towards the work piece. Hard-facing powder 69 of a suitable composition is injected into the plasma column by a carrier gas 64 such as argon, helium, or mixtures of inert gases, through powder-feeding ports in the nozzle 68 onto the work piece. A molten pool forms on the work piece in the arc transfer region that is protected from oxidation or contamination by the shielding gas. Fusion occurs between the deposited powder and the work piece. Direct heating from the plasma provides high density hardfacing which is metallurgically bonded to the work piece. Typical coating conditions are as follows: the arc voltage and current are in the range of about 20-40 volts and about 60-200 amps; the shielding gas flow rates are in the range of about 15-40 standard cubic feet per hour (“SCFH”), and powder feed rates are about 20-150 grams per minute.

[0038] Generally, substrate dilution is about 5% to 15% for hardfacing coatings deposited by a PTA process. “Substrate dilution” is defined as the weight percentage of the substrate metal which has diffused into the binder matrix. Generally speaking, the lower substrate dilution indicates better hardfacing coatings. It should be understood that powder injection is only one way of introducing the hardfacing material into the plasma stream. Any method known in the art is acceptable. For example, an alternative method involves feeding a tube rod of tungsten carbide (approximately 50% by volume and the balance being carbon steel) into the plasma stream, either by hand or by a mechanical process.

[0039] Another acceptable method of applying hardfacing material onto the surface of a cone for erosion protection is the use of a gas-shielding tungsten arc (also known as “gas tungsten arc”) welding process as illustrated in FIG. 7. In this process, an arc is established between a tungsten or tungsten-thorium electrode 72 and a work piece (i.e., cone) 16 which is grounded through welding machine 76. The arc forms a welding pool on the work piece. A hard-facing material in the form of a tube rod 70, which contains approximately 50% tungsten carbide by volume, is fed into the weld pool. The rod 70 is fed either by hand or by a machine. The tungsten electrode 72 is non-consumable. To prevent oxidation and contamination, the heated weld zone, the molten metal and the non-consumable electrode which carries the welding current, are shielded from the ambient atmosphere. They are shielded by an inert gas stream 75 which is directed from the electrode holder 73 through a gas passage 71 to the work piece (i.e., cone) 16. The electrode holder 73 has an electrical conductor that connects the power supply of welding machine 76 to the electrode 72. The electrode holder 73 also includes an insulation sheath 74. The inert shielding gas may include argon, helium or mixtures of these gases. Fusion between the hardfacing material and the cone surface is created by the intense heat of the arc. This heat metallurgically bonds the high density hardfacing material to the work piece. Substrate dilution in this process is generally in the range of about 10% to 20%. The gas-shielding tungsten arc welding process can produce a layer of hardfacing material with a thickness greater than 0.030 inch. Typical coating conditions are as follows: the voltage is about 10-20 volts; the current is about 60-100 amps; and the shielding gas flow rates are in the range of 20-30 SCFH.

[0040] Still another alternative method of applying hard-facing material onto the surface of a cone is the metal inert gas arc (“gas metal arc”) welding process which is illustrated in FIG. 8. In a typical metal inert gas arc system, a welding gun 80 is connected to a power source 81, a control unit 82, and a gas-delivery tubing 90. The welding gun 80 includes a wire 88 which is supplied by a wire reel 83 through wire drive rolls 84. The positive terminal of power source 81 is connected to a work piece (i.e. cone) 16, and the negative terminal of power source 81 is connected to the wire 88 so that an electrical arc (not shown) is generated by passing electrical current between the wire and the work piece. The arc melts the tip of the wire 88, and droplets of the molten wire are subsequently transferred to the surface of the work piece. Contamination of the weld pool by air is prevented by an inert shielding gas 87 which is delivered to the welding gun through the gas-delivery tubing 90. The flow rate of the shielding gas is monitored and controlled by a flow meter 85 and a valve 86. The shielding gas may include any inert gas such as argon, helium, or any mixtures of these gases. In operation, a small-diameter wire 88 is fed from the wire reel 83 to the welding gun 80. The gun 80 has a trigger 89 which operates the wire drive rolls 84, the power supply 81 and the flow of the shielding gas 87. In cases where it is not possible to fabricate flexible wire with a sufficient volume content of tungsten carbide, a straight tube rod could be fed into the welding gun. This feeding process could be manual or mechanized.

[0041] There are four modes of metal transfer in a metal inert gas arc welding process: (1) short circuiting (i.e., dip transfer); (2) globular transfer; (3) spray transfer; and (4) pulsed transfer. In short circuiting (i.e., dip transfer), droplets of molten wire are transferred from the tip of the wire to the work piece by frequently short circuiting the wire to the weld pool with a low current and voltage. This mode of transfer utilizes low heat input which results in a small controllable weld pool. Globular transfer uses somewhat higher currents and voltages than are used for dip transfer. Under this method, metal transfer still occurs by short circuiting the wire to the weld pool. However, spray transfer occurs when the current and voltage are high enough to create free flight of metal droplets with no short circuiting. This provides maximum transfer rates and deep penetration. In pulsed transfer, molten metal droplets are transferred to the surface of the work piece by pulsing the current between a background current and a high pulse current. Typically, the background current is sufficient to sustain the arc but insufficient for substantial metal transfer. However, the high pulse current is set above a threshold level to produce sufficient electromagnetic force for each pulse to transfer one metal droplet from the tip of the wire to the surface of the work piece. As the current is pulsed between the low background current and the high pulse current, the metal droplets are transferred to the work piece successively. Although any pulse frequency may be used, it is preferred that the pulse rate is approximately 50 Hz. Although all four of these modes of metal transfer can be used to deposit hardfacing material on rock bit cones, the pulsed transfer mode is preferred because it provides a higher deposition rate with minimal heat generation and thus results in a higher volume content of tungsten carbide in the hardfacing coating.

[0042] The thickness of the hardfacing material applied to the surface of the cone is generally greater than 0.020 inch, although a preferred thickness is in the range of 0.030 to 0.060 inch. It should, however, be understood that hardfacing coatings with less than 0.020 inch in thickness are also capable of erosion protection, albeit with less efficacy.

[0043] As mentioned above, after the hardfacing material is applied to the cone surface, the cone may then be heat treated before insert sockets or holes are drilled. The heat treatment provides stronger metallurgical bonding, which reduces the likelihood of chipping and flaking off of the hardfacing during operation. Following the heat treatment, the cone insert holes are drilled and the inserts are pressed into the holes and retained with a press-interference fit.

[0044] The hardfacing material used in various embodiments of the invention generally includes a metallic component and a nonmetallic component. The metallic component can be any metal or metal alloys, such as iron, steel, nickel-based alloys, and the like. The non-metallic component generally includes a hard material, such as carbide, boride, and/or nitride. The hardfacing material may be in the form of powder or tube rod, although other forms also are acceptable.

[0045] The volume content of the carbide phase is generally in the range of about 25-60%, with a preferred range of about 35-50%, of the hardfacing material. The carbide phase includes a primary carbide and optionally a secondary carbide. The primary carbide content falls within the range of about 85-95% by volume of the carbide phase. The primary carbide includes single crystal WC, eutectic WC/W₂C, sintered WC/Co, or their combinations. On the other hand, the secondary carbide, which is optional, is the balance of the carbide phase; it is generally in the range of about 5-15% by volume of the carbide phase. The secondary carbide phase includes the following materials: VC, TiC, Cr₃C₂, Cr₇C₃, Cr₂₃C₆ or combinations thereof. As indicated in FIG. 9, the shape of the carbide phase may be angular, irregular, rounded, or spherical. The size of the carbide phase generally is within the range of about 15-500 μm, with a preferred range of about 30-200 μm.

[0046] The volume content of the binder matrix, being the balance of the hardfacing material, generally is in the range of about 35-75% of the hardfacing material. The binder matrix includes a metallic matrix and non-metallic composition. The metallic matrix may contain cobalt, nickel, iron, or mixtures or alloys thereof. It may further include silicon, aluminum, boron, and/or a small amount of refractory metals (such as tungsten, molybdenum, tantalum or other transition metals). The nonmetallic composition includes a secondary carbide and a boride. The total volume content of these materials is between about 7-42%, with a preferred range of about 8-30%, in the binder matrix. The secondary carbides may include VC, TiC, Cr₃C₂, Cr₇C₃, Cr₂₃C₆ or combinations thereof. The borides include CrB, TiB₂, ZrB₂, or combinations thereof. The particle size of the secondary carbides and the borides is between about 10-50 μm. The shape of the particles may be angular, irregular, rounded or spherical. Moreover, the non-metallic composition may further include an Eta phase or a trace amount of oxides which are a by-product of the welding process. Eta phase is a phase of carbides of the formula W₃M₃C or W₆M₆C, where M is Fe, Co, or Ni. The particle size of the Eta phase is generally less than 20 μm, and the particle shape can be crystal-like, irregular, or dendritic.

[0047] Some embodiments concern automating the placement of hardfacing material onto the cone surfaces. This is particularly important when hardfacing material is applied to the cone surfaces in intricate patterns between the inserts. It is critical that deposition of the hardfacing material not interfere with the subsequent insert hole-drilling operation. One method of automation is to use numerically controlled (“NC”) or computer numerically controlled (“CNC”) machines to place the hardfacing material directly onto predetermined areas of a roller cone which are susceptible to erosion. The machines can be programmed using any conventional computer-aided manufacturing techniques to place the hardfacing material sufficiently away from where the insert holes will be drilled. After the cone has been heat treated, the insert holes may be drilled with NC or CNC machines. This will ensure consistency of hole and hardfacing coating locations.

[0048] If a hardfacing material is placed on the cone lands between insert holes, a start mark on the cone may be necessary to ensure proper setup for the hole-drilling process to be synchronized with the hardfacing material deposition process. Other suitable methods to ensure a proper zero or circumferential starting location may also be used. For example, a small start hole in the cone which interfaces with a tooling fixture zero point is one possible method. Another acceptable method is to use a machine with index plates that are timed in phase with the subsequent hole-drilling operation. The machine is set up to place hardfacing material onto the cone surface and automatically index to the next circumferential location. This allows insert holes to be drilled in the intended areas. A start mark is also necessary for proper setup of the hole-drilling operation.

[0049] In some embodiments, only circumferential bands of hardfacing material are deposited in the cone grooves adjacent to the insert lands. It is entirely possible to do this by a robot. Manufacturing parameters such as speed and feed rate may be optimized to achieve the desired hardfacing thickness and consistency.

[0050] To test erosion resistance of rock bit cones coated with the hardfacing material according to the present embodiments, numerous 7-⅞ inch mining rock bits with the hardfacing material applied to the cone grooves were tested in a coal mine. For example, FIG. 9 illustrates a mining rock bit cone that was coated with hardfacing material in the grooves of the cone by the gas-shielding tungsten arc welding process. The process parameters used to hardface the cone are as follows: argon gas flow rate of 25 cubic feet per hour, {fraction (7/16)} inch diameter gas cup, ⅛ inch diameter 2% thoriated tungsten electrode, current of 60 to 80 amps, and voltage of 10 to 12 volts. A tube rod designated as “ST-70 M” was fed into the arc by hand. The 70 M tube rod contained about 65% by weight of macro-crystalline WC with particle size in the range of about 75 to 177 μm and 35% by weight of steel of the AISI 1018 type. The hardfacing coating had a thickness of approximately 0.060 inch and contained approximately 32% of tungsten carbide by volume as the primary carbide. The primary tungsten carbide particles were in the range of 25 to 200 μm with the most common size being approximately 175 μm. The microstructure of the hardfacing coating showed that the tungsten carbide particles were rounded.

[0051] The hardfaced mining rock bits were tested with regular mining rock bits of identical size. Without the hardfacing material, the regular mining bits manifested the primary failure mode—premature loss of the interior inserts near the nose or the apex of the cone. The hard-faced mining rock bits, on the other hand, manifested a significant improvement in cone erosion. Furthermore, premature loss of inserts was virtually eliminated in the case of hardfaced mining rock bits.

[0052] An alternative to the roller cone having hardfacing applied according to the previous embodiments is shown in FIG. 10. The cone 16 includes grooves 50 between insert lands 52, as previously explained, to provide clearance for the cutting elements (inserts) on the adjacent cones on the bit. In this embodiment of hardface application, the hardfacing 51 is applied to the cone surface in the grooves 50 by a welding process. The welding process can include any arc process such as PTA as in the previous embodiments of the invention, or may include conventional torch-type welding according to methods well known in the art. The cone 16 may then be heat treated and sockets 18A for the inserts may be drilled, as in the previous embodiments.

[0053] In the present embodiment, hardfacing 53 is applied to the insert lands 52 using a thermal spraying process after the heat treatment and drilling of the sockets 18A. Thermal spraying processes known in the art include D-gun, HVOF and HAOF. In general, thermal spray hardface application does not raise the temperature of the substrate (the cone body) above about 200 degrees F. (94 degrees C.). The hardfacing applied using thermal spraying can include tungsten carbide of compositions well known in the art.

[0054] To avoid spraying the interior of the sockets 18A, a graphite or similar plug (not shown) may be inserted into each socket 18A prior to thermal spraying. By applying hardfacing using thermal spraying, the insert retention properties of the cone 16 are retained, but the wear susceptible areas of the insert land are covered with hardfacing to reduce erosion of the land surface and consequent insert loss.

[0055] As demonstrated above, the present embodiments are capable of producing highly erosion-resistant hardfacing coatings on rock bit cone surfaces to prevent cone shell erosion during operation. The processes used in the various embodiments of the invention are easy to implement and are cost-effective. Furthermore, the coating thickness, uniformity, porosity and oxide build-up are easier to control than with previous methods. Equally important, the present embodiments provide a hardfacing coating with strong metallurgical bonding between the hardfacing material and the roller cone surfaces where required. This strong metallurgical bonding makes the hardfacing material less likely to chip or flake off during operation. As a result, premature loss of inserts may be virtually eliminated during normal operation of the drill bit.

[0056] While the invention has been disclosed with respect to a number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. For example, although the hardfacing coatings are described in reference to protection of cone erosion in petroleum bits and mining bits, it should be further understood that the invention is equally applicable to other earth-boring devices with rotating elements which experience cone erosion. It should be understood that the invention is applicable to a rock bit with any bearing configuration system, such as friction bearings, sealed bearings, open bearings and the like. As to the composition of the primary carbide, it is preferred that the primary carbide include one or more of single-crystal WC, eutectic WC/W₂C, and sintered WC/Co. It should be understood, however, that any hard carbide may be used in place of single-crystal WC, eutectic WC/W₂C, and sintered WC/Co. Such carbides may include, for example, titanium carbide or chromium carbide. Furthermore, it is also conceivable that a third carbide phase may be beneficial to cone erosion protection. Such a ternary carbide may include any hard carbide materials. Finally, although it is preferred that the hardfacing application occurs before inserts are pressed into the sockets, the invention can be practiced in any other order.

[0057] While the invention has been disclosed with reference to specific examples of embodiments, numerous variations and modifications are possible. Therefore, it is intended that the invention not be limited by the description in the specification, but rather only by the claims that follow. 

What is claimed is:
 1. A roller cone for a rock bit, comprising: a cone substrate; hardfacing applied by a welding process in grooves on the exterior surface of the cone substrate; and hardfacing applied by a thermal spraying process to insert lands on the exterior surface of the cone substrate.
 2. The roller cone as defined in claim 1 wherein the welding process comprises plasma transferred arc welding.
 3. The roller cone as defined in claim 1 wherein the thermal spraying process comprises at least one of HVOF, HAOF and D-gun spraying.
 4. The roller cone as defined in claim 1 wherein the hardfacing applied by at least one of the welding process and thermal spraying process comprises tungsten carbide.
 5. A roller cone bit, comprising: a bit body adapted to be coupled to have a drill string and having at least one journal depending therefrom; a roller cone rotatably mounted on the at least one journal, the roller cone comprising a cone substrate, hardfacing applied by a welding process in grooves on the exterior surface of the cone substrate and hardfacing applied by a thermal spraying process to insert lands on the exterior surface of the cone substrate; and cutting elements affixed to the roller cone at selected positions thereon.
 6. The roller cone bit as defined in claim 5 wherein the welding process comprises plasma transferred arc welding.
 7. The roller cone bit as defined in claim 5 wherein the thermal spraying process comprises at least one of HVOF, HAOF and D-gun spraying.
 8. The roller cone bit as defined in claim 5 wherein the hardfacing applied by at least one of the welding process and thermal spraying process comprises tungsten carbide.
 9. A method for hardfacing a roller cone for a drill bit, comprising: applying hardfacing by a welding process to grooves between insert lands on a roller cone body; and applying hardfacing by a thermal spraying process to the insert lands.
 10. The method as defined in claim 9 wherein the welding process comprises plasma transferred arc welding.
 11. The method as defined in claim 9 wherein the thermal spraying process comprises at least one of HVOF, HAOF and D-gun spraying.
 12. The method as defined in claim 9 further comprising heat treating the cone body and drilling insert sockets therein prior to the applying hardfacing to the inset lands.
 13. The method as defined in claim 9 wherein the hardfacing applied by at least one of the welding process and thermal spraying process comprises tungsten carbide. 